As we move into the autumn and winter months, we start to see an increase in fog occurrences under particular weather conditions.
Fog is defined as water droplets that are suspended in the atmosphere near the earth’s surface, reducing the visibility to less than one kilometre.
Between September and February in the Aylesbury Vale, we might expect to see anything from 10 to 15 days of fog, but potentially more in some spots.
There are two main mechanisms which cause fog to form; advection and radiation.
Advection fog occurs where a moist air mass moves over a colder surface (often cold water such as the sea), causing the air to cool and condensation to form.
However, it is radiation fog that is the most commonly observed type of fog and is responsible for the vast majority of fog events in the Vale.
In this mechanism, the earth’s surface cools overnight which then cools the layer of air above the surface, again leading to condensation.
For fog to form in this way it is important that winds are light so that warmer air from higher levels of the atmosphere are not mixed down into the surface layer.
The conditions that are most conducive to radiation fog formation are generally found where high pressure is dominant, leading to light winds and periods of clear skies, allowing more cooling at night.
The end of this week is looking mostly dry with some sunny spells.
Throughout Thursday it is possible that there may be a few showers.
As we head towards this weekend and into next week, it is looking mostly dry with high pressure currently forecast to build over the south of the country leading to some good periods of sunshine, chilly nights, but also the chance of some fog.